Disclosed are a flexible display panel, a flexible display device and a deformation detection method thereof, used for detecting deformation of the flexible display panel. The flexible display panel provided by embodiments of the present disclosure includes a flexible substrate, a display device and a piezoelectric sensor arranged in a stacked mode. The piezoelectric sensor includes a first electrode, a second electrode, and a piezoelectric layer positioned between the first electrode and the second electrode; the piezoelectric sensor is configured to generate an electrical signal under the action of stress produced by bending the flexible display panel; and a signal processing chip in the flexible display device is configured to determine deformation parameters of the flexible display panel according to the electrical signal.